1. Field of the Present Invention
The present invention relates to the field of motion control and in particular to control loops including proportional and integral control factors which are used to control the rotational speed of motors such as the capstan drive motors used in magnetic tape drives.
2. Description of the Prior Art
Feedback or control loops are used to control many industrial processes, such as the rotational speed of motors. Simple control loops include a set point or desired value input, a measurement input which indicates the actual value of the parameter to be controlled, and a comparator to develop an error signal related to the difference between the desired and actual values. A control loop output signal, related to the error signal, is then applied to the control device whose parameter is to be controlled, such as a motor whose speed is to be controlled by the loop.
The control accuracy and response characteristics of control loops are conventionally enhanced by adding various control terms or weightings to the error signal in order to develop the control output signal. One classic enhanced servo control loop is known as the PID loop which includes proportional, integral and derivative terms added to the error signal to develop the desired control signal. PID loops are often applied where the accurate maintenance of a controlled parameter is important, such as the control of the rotational speed of the capstan in a magnetic tape drive.
The conventional proportional control term is a linear gain factor related to the difference between the magnitude of the error signal and the magnitude of the control signal necessary to achieve the desired result. The conventional integral term is a long time constant linear gain term, related to the integral of error signal, used to reduce the residual error that would otherwise occur in a proportional only control loop between the setpoint and measured values. Although integral terms are used to slowly bring the parameter to be controlled to exactly equal the desired setpoint value, integral terms tend to degrade system response to short term transients. Conventional derivative terms, related to the derivative of the error signal, are added to enhance system response to such short term transients without reducing the long term accuracy benefits of the integral terms.
Conventional control loops may be implemented in either hardware or software. For example, in a conventional hardware implementation, the PID terms are provided by separate amplifier, integrator and differentiator circuits. Alternately, these terms may be applied in a software implementation by an appropriate algorithm in a computer used to generate a value for the control signal in response to applied values for the measurement and setpoint inputs.
One common problem with conventional control loops, known as integral wrap-up, results from the inherent non-linearities of such control loops exaggerated by the non-linearities of the components of the loop when they are operated outside of their linear range. In a conventional capstan speed control loop, the amplifier used to apply the control voltage to the capstan drive motor is very non-linear in that the amplifier can drive the motor speed up very quickly but it cannot reduce motor speed as quickly because it must rely on system losses to reduce the speed. Wrap up is an exaggerated integral error which is built up during transient conditions, such as start-up or when the motor is temporarily jammed or stalled by an external problem. After startup is accomplished, or the stalled or jammed condition is resolved, the resultant built up error in the integral term hampers accurate control rather than enhancing it.
One known technique for reducing the harmful effects of integral wrap up is the feedforward technique in which closed loop servo control is sacrificed at start up for a predicted open loop control dependent upon the expected system response. Another known technique for reducing integral wrap up is to limit the integral value to a known, safe limit. That is, when a predetermined safe limit of integral error is reached, such conventional systems restrict further growth of the integral error term to a safe value which will not interfere with normal system operation, but will reduce the drastic overshoots which would otherwise occur after the transient condition has been removed.
For example, in a magnetic tape drive capstan speed control loop, if the tape is temporarily jammed by a piece of grit, the integral error builds up during the jam and causes an undesirable control response when the grit is dislodged. By limiting the integral build up to a predetermined safe limit, the undesirable control response can be limited.
Such known techniques for correcting or limiting integral wrap up require the relative complex determination of load dependent variables and are therefore not completely satisfactory. What is needed is a technique for limiting the harmful effects of integral wrapup without sacrificing closed loop control accuracy and response characteristics or requiring complex load dependent variable determinations.
As will become apparent in the "Detailed Description of the Invention" section below, the present invention overcomes many of the above disadvantages of the prior art in solving the problem of wrap up in PID loops.